This invention relates generally to laser spectroscopy, and more particularly the invention relates to frequency modulation laser spectroscopy.
Advances in laser technology have created powerful new tools for spectroscopy. Frequency modulation (FM) laser spectroscopy is a sensitive optical spectroscopy technique for measuring absorption and dispersion in an optical medium. As discussed by Cooper and Gallagher in "Double Frequency Modulation Spectroscopy: High Modulation Frequency Low Bandwidth Detectors", Applied Optics, Vol. 24, No. 9, 1 May 1985, Pages 1327-1334, one of the most promising areas for the application of FM spectroscopy is in the detection of atmospheric trace gases and hazardous materials. Absorptions as small as 10.sup.-4 have been easily detected with FM spectroscopy using either single mode or multimode CW lasers, and an optimized visible wavelength FM system should be capable of detecting an absorption as small as 10.sup.-6 with a one second integration time.
In FM laser spectroscopy a laser beam of frequency .omega..sub.o is phase modulated at frequency .OMEGA. which is usually greater than the line width .DELTA..omega..sub.o of the laser. Typical values are .OMEGA.=500 MHz and .DELTA..omega..sub.o =1 MHz. In the limit of low modulation index, the laser beam acquires sidebands at .omega..sub.o .+-..OMEGA. and when the modulated laser beam impinges on a square law detector, such as a photodiode, each sideband beats with the carrier to produce a component of the photo current at .OMEGA.. When there is no absorption the two beat signals are 180.degree. out of phase and therefore cancel. If prior to photodetection the modulated beam traverses a medium whose complex index of refraction differs for the two sidebands, the sideband cancellation is incomplete and a photocurrent at .OMEGA. is produced.
In generating the FM sideband, Cooper and Gallagher modulate a laser beam using two modulators, one modulator frequency being 2.OMEGA.+.sigma. and the other modulation frequency being .OMEGA.. The circuitry is somewhat complex in the number of components and in optical and electronic alignment.